The invention relates generally to a convenient system and method for efficient diagnosis and servicing of engine system problems in automobiles and other vehicles that employ feedback-loop engine management computer systems. The system and method take advantage in a novel way of the characteristic electronic "signatures" that are generated by various components of an engine management computer system, particularly the oxygen sensor that monitors oxygen levels in engine exhaust gases. In addition, the system and method advantageously permit efficient and reliable detection of malfunctioning oxygen sensors, reducing the likelihood that a properly functioning sensor or other component will be wastefully replaced for lack of correct diagnostic information. The method has a wide application in that it may be utilized with many if not all vehicles which possess oxygen sensor feedback-loop engine control systems.
1.1. Vehicle Oxygen Sensors
As is well known to those of ordinary skill in the field of servicing computer controlled vehicle engines oxygen sensors (depicted in FIG. 1 by reference numeral 101) are commonly built into modem vehicle exhaust systems to monitor engine exhaust gases. As the air-fuel mixture ratio introduced into the engine cylinders changes, the quantity of oxygen (O.sub.2) in the exhaust changes. The oxygen sensor 101 emits a voltage which is related to the amount of oxygen in the exhaust and the specific design of the sensor.
Several types of oxygen sensors are currently used in computerized vehicle emission control systems. Zirconium dioxide sensors, commonly known as zirconia sensors, are perhaps the most common and are found with and without heating elements. Zirconia sensors generate a voltage when heated by exhaust gases in an oxygen-deficient atmosphere. These sensors have a nominal electrical output that typically ranges from zero to one volt, dependent on the oxygen content of the exhaust. More recently, titanium based sensors have been employed for essentially the same purpose as their zirconia predecessors, except that as the O.sub.2 level changes, the resistance across the sensor changes. When a reference voltage is applied to the sensor, the sensor returns a voltage to the computer which is directly related to the O.sub.2 level in the exhaust.
1.2. Variation of Fuel-Air Mixture for Efficient Catalytic Conversion
The primary use of O.sub.2 sensor information by vehicle engine management computer systems is in stoichiometric control of the fuel-air mixture introduced into the engine cylinders to aid catalytic conversion of the engine exhaust gases. The catalyst in most catalytic converters works most efficiently and lasts longer when subjected to a slight excess of air, followed by a slight excess of fuel, and so forth, as opposed to being subjected to a predominantly, non-oscillating mixture. Accordingly, the engine management computer system (sometimes referred to as an engine control module or ECM, identified in FIG. 1 by reference numeral 103) generates control signals to devices which alter the fuel-air mixture, e.g., fuel injectors. More specifically, the ECM 103 receives a voltage signal from the oxygen sensor 101 via an oxygen sensor lead 110. As noted above, that voltage is a function of the oxygen content of engine exhaust gases. The ECM 103 utilizes the voltage signal to vary the fuel-air mixture injected into the cylinders.
1.3. The Problem of Servicing Malfunctioning O.sub.2 Sensors
Plainly, an ECM cannot optimally control its engine's fuel-air mixture if the O.sub.2 sensor is malfunctioning. The Environmental Protection Agency (EPA) has stated that a large portion of engine emissions-test failures (i.e., engines that produce excessive pollutants) are due to malfunctioning oxygen sensors, by some estimates up to 50% of such failures.
In the field of vehicle servicing, however, no feasible or economical test is known to exist for determining whether an oxygen sensor is in fact malfunctioning. As a result, unnecessary replacement of oxygen sensors and other parts frequently occurs because of erroneous diagnosis and/or guesswork on the part of mechanics. It has been reported by oxygen sensor manufacturers that a large portion of all supposedly defective oxygen sensors that are returned to the manufacturer under warranty are in fact not defective.
Part of this diagnosis problem arises from the ECM's variation of the fuel-air mixture. Typically, an efficient predominantly fuel-air mixture results in an oxygen sensor output voltage averaging about 0.45 volts. By the same token, an efficient oscillating mixture likewise has an average voltage of about 0.45 volts. Use of a conventional volt-ohm meter (VOM), whether analog or digital (DVOM), cannot easily detect oxygen sensor problems because their sampling rates and averaging circuits do not give accurate representations of voltage vs. time. Due to the same limitations, scan tools are also inadequate.